1. Field of the Invention
The present invention relates to a Cr-containing steel. In particular, the present invention relates to a soft Cr-containing steel which has both heat resistance and formability and is suitable for members used in high-temperature environments, for example and especially, exhaust pipes of automobiles and motorcycles, outer casings for catalysts, and exhaust ducts in thermal power plants.
2. Description of the Related Art
Exhaust system members such as exhaust manifolds, exhaust pipes, converter cases, and mufflers, used in exhaust environments of automobiles are required to have superior formability and superior heat resistance. Conventionally in many cases, Cr-containing steel sheets containing Nb and Si, for example, Type 429 (14Cr-0.9Si-0.4Nb-base) steel, which is malleable, and has superior formability at room temperature, and has relatively increased high-temperature strength, have been used for the aforementioned applications. However, when exhaust gas temperatures are increased to about 900xc2x0 C., which is higher than can be endured due to improvements of engine performance, there is a problem in that Type 429 steel has an insufficient high-temperature proof stress.
In order to solve the aforementioned problems, SUS 444 (JIS (Japanese Industrial Standard) G4305, 19Cr-0.2Nb-1.8Mo) steel, which is a Cr-containing steel having improved high-temperature proof stress by addition of Nb and Mo, has been developed. However, there were problems in that the SUS 444 steel was expensive to produce because of the large amounts of alloying elements required, and in particular, molds were significantly worn during use due to high yield strength YS. Furthermore, although a technique related to a stainless steel having superior intergranular corrosion resistance, superior formability into pipes, and superior high-temperature strength has also been disclosed in Japanese Unexamined Patent Application Publication No. 4-228547, since malleability at room temperature was not taken into consideration, there has been a problem in that molds were significantly worn during use.
Accordingly, requirements for a material having strength equivalent to, or less than, that of Type 429 steel and malleability with excellent workability at room temperature, and having superior high-temperature strength, in order to have a proof stress higher than that of Type 429 steel at 900xc2x0 C., have become even more intensified. When the high-temperature strength of the material for the exhaust system members is increased, it becomes possible to reduce the thicknesses of the members so as to contribute to reduced weight of automobile bodies. As a consequence, requirements for an increase in high-temperature strength have become even more intensified. Furthermore, accompanying the increase in exhaust gas temperatures, a material for exhaust system members has also been required to have superior oxidation resistance in order to prevent the occurrence of undesired oxidation at high temperatures.
For example, in Japanese Unexamined Patent Application Publication No. 2000-73147, a Cr-containing steel having superior high-temperature strength, workability, and surface properties has been disclosed as a raw material which can be applied to a wide range of temperatures from the high temperature portion to the low temperature portion of the exhaust system member. This raw material is a Cr-containing steel containing C: 0.02% or less, Si: 0.01% or less, Cr: 3.0% to 20%, and Nb: 0.2% to 1.0%, and precipitation of the Fe2Nb Laves phase is prevented in order to prevent an increase in yield strength at room temperature, and to impart superior high-temperature strength and workability, as well as excellent surface properties.
However, there was a problem in that the steel described in Japanese Unexamined Patent Application Publication No. 2000-73147 could not sufficiently satisfy the properties recently required of the material for exhaust system members, since, for example, undesirable oxidation occurred when the steel was heated to a high temperature in the region of 900xc2x0 C.
The present invention was made to solve the aforementioned problems in the conventional techniques and to provide advantages. Accordingly, it is an object of the present invention to provide a soft Cr-containing steel having malleability and superior workability at room temperature, and having, especially, superior high-temperature strength compared to those of conventional steels, as well as superior oxidation resistance. Herein, xe2x80x9cmalleable at room temperaturexe2x80x9d means that when the steel is produced under the same conditions as the conventional steels, such as type 429, a strength equivalent to, or less than, those of the conventional steels can be achieved, xe2x80x9csuperior high-temperature strengthxe2x80x9d means that a proof stress (0.2% PS) at 900xc2x0 C. is 17 MPa or more, and xe2x80x9csuperior oxidation resistancexe2x80x9d means that undesired oxidation does not occur at 900xc2x0 C.
In order to achieve the aforementioned objects, the inventors of the present invention earnestly researched regarding a composition that can significantly improve high-temperature strength without an increase in room-temperature strength of a Cr-containing steel containing Nb. As a result, the inventors of the present invention discovered that regarding the composition, when the Si content was limited to within an appropriate minimum range, the Mo content was appropriately specified in connection with the Si content, and the Cr content was reduced as much as possible, precipitation of the (Fe,Cr)2(Mo,Nb) Laves phase was prevented and Mo was present primarily in the form of solid solution Mo, and therefore, the Cr-containing steel had malleability at room temperature, and had a significantly improved strength at high temperatures, and the occurrence of undesired oxidation could be prevented.
That is, according to the present invention, a soft Cr-containing steel having a composition composed of, on a % by mass basis, C: from about 0.001% to about 0.020%, Si: more than about 0.10% and less than about 0.50%, Mn: less than about 2.00%, P: less than about 0.060%, S: less than about 0.008%, Cr: from about 12.0% to about 16.0%, Ni: from about 0.05% to about 1.00%, N: less than about 0.020%, Nb: from about 10xc3x97(C+N) to about 1.00%, Mo: more than about 0.80% and less than about 3.00%, and Fe and incidental impurities, wherein the contents of alloying elements, silicon and molybdenum, represented by Si and Mo, respectively, on a % by mass basis, satisfy the following formula (1), could be achieved.
Sixe2x89xa61.2xe2x88x920.4Mo.xe2x80x83xe2x80x83(1) 
In the present invention, the aforementioned soft Cr-containing steel preferably further contains, on a % by mass basis, at least one selected from the group consisting of Cu: from about 0.05% to about 1.00%, Ti: from about 0.02% to about 0.50%, V: from about 0.05% to about 0.50%, and B: from about 0.0005% to about 0.0100%. The aforementioned soft Cr-containing steels preferably further contain W: from about 0.50% to about 5.00% by mass. The aforementioned soft Cr-containing steels preferably further contain Al: from about 0.02% to about 0.50% by mass. The aforementioned soft Cr-containing steels preferably further contain, on a % by mass basis, at least one selected from the group consisting of REM: from about 0.03% to about 0.10% and Zr: from about 0.05% to about 0.50%.
In the present invention, regarding the state of Mo in the steel, a ratio of (112) diffraction intensity of the Laves phase, (Fe,Cr)2(Mo,Nb), to (111) diffraction intensity of Nb carbonitride, Nb(C,N), A value=I{(Fe,Cr)2(Mo,Nb)}(112)/I{Nb(C,N)}(111), is preferably less than 0.4 based on X-ray diffraction of extraction residues of precipitates in the steel.